The invention relates to radio frequency tuners, and more particularly to such tuners which employ phase-locked-loops as frequency generation elements, and most particularly to phase detectors used in such phase-locked loop radio frequency tuners.
Phase-locked-loops are commonly used in circuits to produce a precise, stable frequency. They can be used both as an oscillator in a receiver to detect signals of prescribed frequency and as a frequency synthesizer in a transmitter to generate an output signal having a selectively variable frequency. The basic elements of a phase-locked-loop (PLL) include a voltage controlled oscillator (VCO) for producing an output signal having a controlled frequency, a detector for comparing the phase of the output signal with that of a predetermined reference signal and for producing an error signal representing the detected phase difference, and a loop filter for filtering the error signal and coupling it to the VCO to controllably adjust the output signal's frequency.
A common use of PLL frequency synthesizers is in frequency hopping radio communications systems which are used for secure communications. These systems use radio transmitters and receivers which hop in synchronization from one frequency to another in a predetermined sequence, transmitting and receiving a small part of a message on each frequency. The security of the frequency hopping technique depends both upon the choice of frequencies and upon the hopping rate. Increasing the hopping rate increases the difficulty of unauthorized monitoring stations in determining the hopping patterns and following the hopping sequence.
The frequency hopping rate is limited primarily by the time required for PLL's to change from one frequency to another. The frequency change rate is affected by a number of factors. For example, the bandwidth of the PLL loop filter significantly limits the frequency change rate. More relevant to the present invention, however, are the limitations due to the phase detector.
Prior art phase detectors of the differential type are subject to "cycling" outside the desired phase lock range as the phase relationship between the reference signal and the VCO output signal changes. This results in an error signal which varies with phase angle, and over several 2.pi. phase ranges exhibits a triangular, "sawtooth" waveform. It is the average level of this signal which is used to urge the VCO toward the desired lock-in range. With the triangular waveforms of prior art detectors, this voltage, and the resulting slew rate, is only 50 percent of theoretical due to cycling. The VCO output frequency thus changes more slowly than if the peak signal level were used.
Prior art phase detectors are also subject to an ambiguity since it is not possible to determine from the cycling phase detector error signal whether the PLL is within or without the desired lock-in range and thus a momentary false lock-in indication is given. Consequently, there is no positive indication of lock-in, and a transmitter cannot be limited to operation only while positively locked onto the selected frequency.
It is therefore an object of the present invention to provide an improved PLL.
It is another object of the present invention to provide a PLL which locks onto the desired frequency more quickly.
It is a further object of the present invention to provide a PLL which permits operation of a radio transmitter only when the PLL is locked onto the desired frequency.
It is an additional object of the present invention to provide a PLL which provides a smoother change from one frequency to another.
It is still another object of the present invention to provide an improved phase detector for use in PLL's.
It is still a further object of the present invention to provide a phase detector which provides a higher level error signal for control of a VCO, outside the desired lock-in range.
It is an additional object of the present invention to provide a phase detector which provides a positive lock-in indication.